The present invention relates to an aqueous cementing composition and method of using same in cementing oil and gas wells and the like. More particularly, the present invention concerns incorporation of at least water dispersible terpolymers or salts thereof prepared in part from a monomer which is an unsaturated polybasic acid as a fluid loss control additive in a hydraulic cement used in drilling wells in subterranean formations. 2. Description of Art
Oil well cementing and other underground cementing operations often require placement of a slurry of cement, water and other additives in a porous environment such as porous earthen or rock strata. For example, cement compositions are used in the oil and gas industry to cement the annular space in the well bore between the surrounding formation and pipe or casing. Typically, the cement slurry is pumped down the inside of the casing and back up the outside of the casing through the annular space. The slurry is allowed to set up or harden in the annular space, thereby forming a rigid column which ideally forms a bond with the earth formation as well as with the metal pipe. To achieve a satisfactory primary cementing job, it is important to achieve a tight bond to prevent vertical communication of fluids or gas along or within the column, which could contaminate the producing zone, or permit a loss of reserves.
The primary functions of the cementing process are to restrict fluid movement between geological formations and to bond and support the casing. In addition the cement aids in protecting the casting from corrosion, preventing blowouts by quickly sealing formations, protecting the casing from shock loads in drilling deeper wells, and sealing off lost circulation or thief zones.
A common problem in petroleum well cementing is the flow of liquid from the cement slurry into porous earth formations in contact with the cement. This fluid loss is undesirable since it can result in dehydration of the cement slurry, and it causes thick filter cakes of cement solids which can plug the well bore. The fluid lost can damage sensitive formations. Cement fluid loss is particularly a problem in the process known as squeeze cementing.
Problems develop when water filters out of the slurry, i.e., the filtrate, into the porous media during the setting period. As a result of the rapid water loss, the cement acquires impaired qualities of strength and an uncontrollable setting rate. Also, the water loss from the cement frequently damages the surrounding strata formation. The problem is not cured by producing a cement slurry containing more water. This results in serious placing and setting problems.
Adequate fluid loss control is also important in achieving effective primary cementing. Inadequate fluid loss control can result in the formation of a bridge in the annulus opposite a permeable zone, thus isolating a lower zone from the hydrostatic pressure above the bridge. Only a small amount of filtrate loss beneath such a bridge is then necessary to drop the annular pressure to beneath that of the formation pressure. The result is an influx of formation fluids and pressure, creating flow channels and the need for expensive remedial work.
In attempting to control fluid loss from the cementing slurry to the surrounding formation, it is important to reduce the cement matrix permeability and retain water during the initial set, effectively blocking the porous cement structure. One way to do this is to reduce filtrate mobility by increasing the filtrate viscosity to counter the normal thermal thinning of the cement slurry which occurs at down hole temperatures. An increase in filtrate viscosity at down hole temperatures minimizes thermal thinning and increases the retention of the filtrate within the cement matrix. Conventional fluid loss polymers do not effectively counteract thermal thinning with increased temperature thereby allowing fluid loss to the formation and promoting stratification of solids within the cement column.
This is a requirement, therefore, for materials which, when added to the cement formulation, reduce the loss of fluid from the slurry to porous formations.
Certain polymer compositions have long been recognized by those skilled in the art of cementing wells in the petroleum industry as cementing additives useful in reducing fluid loss from a slurry of cement and water to the surrounding environment, i.e., the formation. These compositions are commonly referred to as "fluid loss additives." Such additives will be used in well cementing operations where the bottom hole circulating temperatures (BHCT) may range from 80.degree. to 170.degree. F., substantial salt concentrations may be present, and slurry retardation and viscosity are critical aspects as same affect pumpability and compressive strength. Exemplary prior art fluid loss additives are described in U.S. Pat. Nos. 3,140,269; 3,234,154; 3,483,007; 3,491,049; 3,662,830 and 4,258,790.
One type of fluid loss additive used in cementing compositions consists of cellulose-base controlling agents, such as methyl cellulose, carboxymethylcellulose (CMC) and hydroxyethylcellulose (HEC) which may be employed with or without a dispersant such as condensed napthalenesulfonic acid salts. However, there are several disadvantages to the use of CMC or HEC as cement fluid loss control additives. They are solid and as a result are difficult to handle in offshore operations. In addition, they tend to considerably increase the slurry viscosity, retard the cement, thereby preventing its movement under turbulent flow conditions. Also, they lose effectiveness in the presence of soluble calcium salts and at elevated temperatures.
Another type of fluid loss control additive exhibiting desirable qualities has been polymers such as polyamines and polyethyleneimines. U.S. Pat. No. 3,491,049 to Gibson et al. discloses an aqueous hydraulic cement slurry including hydraulic cement, water, a surfactant, and a small amount of polyalkylenepolyamine, polyalkenimine or a mixture thereof. The Gibson et al. patent teaches the use of a sulfonated naphthalene condensate dispersant as an additional additive to the cement slurry which cooperates with the polyamine additive to provide satisfactory fluid loss in cement slurries used at about 200.degree. F. and below. The sulfonated naphthalene dispersant is typically a low molecular weight material, e.g., in the range from about 1,000 to 3,000. McKenzie et al. discuss use of liquid polyamines with lignosulfonate in their article entitled "Polyamine Compounds Control Cement Fluid Loss in Freshwater of Seawater Slurries", Oil & Gas Journal, 80(13) Mar. 29, 1982, pp. 146-148.
Use of polyamines as an additive has some serious drawbacks however. The polyamines are available only as liquids, usually as 30-50 percent active solutions of the polymers in water, and give very thin slurries. Consequently, they can be added to the hydraulic cement only at the time of formation of the cement slurry. This must be done at the well site immediately prior to pumping the slurry into the well. Under these circumstances, the liquid polyamines must be available at the well and measured for mixing with the cement under non-ideal conditions. Their main disadvantage is that they tend to be incompatible with most of the additives for cement, such as dispersants and retarders, due to their cationic nature.
The liquid polyamines also require addition of sulfonated polymers to the cement composition to provide effective fluid loss control. It has been known that sulfonated polymers form a slimy pultaceous precipitate with the polyamines. This is disclosed in U.S. Pat. No. 2,839,417 to Tousignant et al. This precipitate is the effective fluid loss control agent. The sulfonated polymers have the characteristic of acting as retardants. This characteristic is undesirable at temperatures below 100.degree. F.
Another cement additive employed for control of fluid loss is a copolymer of acrylamide and acrylic acid [L. F. McKenzie, F. M. McElflesh, SPE 1-623,279 (1982)]. Although this material performs well at high temperature and in the presence of soluble calcium salts, it has the undesirable property of strongly retarding cement. This retarding effect increases with increasing temperature, most likely due to the hydrolysis of the unstable amide groups contained in the polymer which accelerates at elevated temperature. Hydrolysis of amide functions produces additional carboxylic acid residues which are credited with cement retarding activity.
Mixtures of HEC, polyvinyl pyrrolidone and sodium naphthalene sulfonate are described as cement fluid loss additives in U.S. Pat. Nos. 3,132,693 and (3,359,225) describes mixtures of polyvinyl pyrrolidone and sodium naphthalenesulfonate-formaldehyde condensation products as cement fluid loss additives. Also complex mixtures of maleic anhydride-N-vinylpyrrolidone copolymers with polymers of poly (aryl-vinylbenzyl) alkyl and hydroalkyl substituted quaternary ammonium bases and salts are described as cement fluid loss additives in U.S. Pat. No. 3,140,269 to Wahl. An improved fluid loss additive for well cements consists of 30 to 70 weight percent of N-vinyl-pyrrolidone homopolymer, from about 5 to 19 weight percent of carboxymethyl cellulose or hydroxyethylcellulose, and the sodium salt of condensed naphthalenesulfonic acid. [B. W. Hale, U.S. Pat. No. 4,258,780 (Mar. 31, 1982)].
U.S. Pat. No. 3,409,080 to Harrison, issued Nov. 5, 1968, mentions that polyvinyl alcohol and polyvinyl acetate can be used as polymeric fluid loss agents in oil well cements. The particular type of polyvinyl alcohol utilized is not stressed and the example given in the patent uses a polyvinyl alcohol which is 88% hydrolyzed.
It is also applicants' understanding that the polyvinyl alcohol polymers used as fluid loss agents in the past were always soluble in the cement slurry at ambient temperatures.
Another polymer described in U.S. Pat. No. 4,015,991 to Persinski et al. as a useful fluid loss additive for hydraulic oil well cements consists of a hydraulic cement slurry consisting of hydrolyzed copolymers of acrylamide (AM) and 2-acrylamido-2-methyl propane sulfonic acid (AMPS). However, these AM/AMPS copolymers are useful only in operations where the bottom hole circulation temperature (BHCT) ranges from 90.degree. to 125.degree. F., whereas BHCT ranges encountered in such operations are often outside such a range. Still further, these copolymers have a salt tolerance of only up to about 10%.
The temperature limitations of the AM/AMPS copolymers, i.e., loss of usefulness above about 125.degree. F. BHCT, are believed to be the result of hydrolysis of the amide groups. The carboxylate groups formed by such hydrolysis convert the copolymers to materials which function to retard the setting of the cement and to reduce the compressive strength of the set cement. Further, in the lower portion of the above-mentioned temperature range (between 90.degree. and 100.degree. F.) the AM/AMPS is less effective as a fluid loss additive, requiring inclusion of larger amounts of such additive than at higher temperatures. The inclusion of sufficiently large amounts of additive to create an acceptable fluid loss composition often creates viscosity and pumpability problems, since the addition of such copolymer directly affects the resultant slurry rheology. Copolymers of acrylamide and AMPS exhibit high viscosity and poor mixability, resulting in cement slurries having poor pumpability characteristics during cementing operations. Mixability is a subjective term used to describe how well the components in the cement composition wet and mix with each other, as well as the energy required to create a generally homogeneous slurry.
Hence, the industry desires a fluid loss additive that has as little effect on compressive strength, set time, viscosity and thickening time as possible; is salt tolerable, i.e., does not exhibit substantial loss of effectiveness in the present of salt; and is chemically stable during cementing operations. Further, such desired fluid loss additive should be compatible with as many other additives and environmental conditions as possible, should be water dispersible or soluble in cement slurries at normal ambient temperature encountered in oil well cementing operations, as well as to continue to provide fluid loss characteristics over broad temperature and cement pH range.
U.S. Pat. No. 4,404,111 discloses the use of copolymers of N,N-dimethylacrylamide (NNDMA) and AMPS as viscosity control agents in aqueous compositions to facilitate petroleum recovery from subterranean bearing formations. The method of preparing said copolymers uses conventional free radical initiators such as ammonium persulfate and results in copolymers having average molecular weights of greater than about one million. Further, the amount of NNDMA monomer employed in preparing the AMPS/NNDMA copolymer is disclosed as between 70 and about 99.5 weight percent.